1. Field of the Invention
The present invention relates to a digital TV, and more particularly, to a VSB receiver and a carrier recovery apparatus thereof, in which a carrier is recovered using a VSB modulated signal.
2. Discussion of the Related Art
A vestigial sideband (VSB) system of Grand Alliance is adopted as the standard for a transmission system of digital TV (e.g., HDTV) in the United States and Korea. In the VSB system, when a signal is amplitude-modulated, upper and lower sidebands are generated based on a carrier wave. At this point, when one of the two sidebands is greatly reduced, the other sideband is modulated. That is, only one sideband spectrum of a baseband signal is removed to a passband and then transmitted, such that the VSB modulation is more efficient in the use of bandwidth.
In the VSB modulation, if a DC spectrum of a baseband signal is removed to a passband, the DC spectrum is converted to a tone spectrum. This signal is called a pilot signal. That is, when a broadcasting station performs the VSB modulation, the pilot signal is carried and transmitted together via air so that a receiver can correctly demodulate the signal.
FIG. 1 is a schematic block diagram of a general digital TV transmitter. Referring to FIG. 1, a randomizer 101 randomly outputs an input data to a Read-Solomon (RS) encoder 102. The RS encoder 102 performs an RS encoding of the randomly inputted data for inner and outer channel coding and adds a parity code of 20 bytes to outputs the resulting signal to an interleaver 103.
The interleaver 103 interleaves the RS encoded data according to a preset regulation and outputs the interleaved data to a trellis encoder 104. The trellis encoder 104 converts the interleaved data into a symbol in byte for a trellis coding and outputs it to a multiplexer 105.
The multiplexer 105 performs a multiplexing of a segment synchronizing signal and a field synchronizing signal to a trellis coded symbol sequence per segment and frame to thereby form a frame, and then outputs the frame to a pilot insertion part 106. The pilot insertion part 106 inserts a pilot signal of a DC value into the framed transmission symbol and outputs to a VSB modulation part 107.
The VSB modulation part 107 modulates the symbol sequence having the pilot signal inserted thereinto in the VSB system and outputs the modulated symbol sequence to an RF up-converter 108. The RF up-converter 108 converts the VSB signal of the modulated baseband into an RF passband signal so as to effectively transmit the signal via an antenna.
FIG. 2 is a block diagram of the VSB modulation part of the digital TV transmitter shown in FIG. 1.
Referring to FIG. 2, a channel encoder 201 is configured with the randomizer 101, the RS encoder 102, the interleaver 103, the trellis encoder 104, the multiplexer 105, the pilot insertion part 106, and the VSB modulation part 107 includes a complex filter 202 and an intermediate frequency (IF) modulator 203. A signal passes through the channel encoder 201 and the VSB modulation part 107. First, for the VSB modulation, the signal passing through the channel encoder 201 passes through the complex filter 202. At this point, if the encoded digital signal passes through the complex filter 202, a Hilbert transformer and an SRC transforms the shapes of frequencies of I and Q signals such that the I and Q signals can be VSB-modulated.
The I and Q signals as the output of the complex filter 202 are IF-modulated at the IF modulator 203 and then subtracted at a subtracter 204, resulting in a VSB IF signal of a required bandwidth (6 MHz). In order to transmit the VSB IF signal via radio wave, the VSB IF signal is converted into an RF passband signal by the RF up-converter 205 and the RF baseband signal is transmitted via the antenna.
FIG. 3 is a block diagram of a general digital TV receiver based on the ATSC standard.
In the digital TV receiver, a passband signal of a specific channel is extracted by a tuner 302 and a carrier wave is recovered using a pilot signal inserted into a sideband. Then, a transmission symbol is extracted from the recovered baseband signal by a symbol timing recovery and a channel compensation.
Referring to FIG. 3, the digital TV receiver includes: a tuner 302 for selecting a desired channel frequency from the RF signal received through an antenna 301 and primarily converting the VSB signal from the RF band to an IF band; a surface acoustic wave (SAW) filter 303 for passing a predetermined band of the IF signal output from the tuner 302; an IF processing part 304 for secondarily converting an output signal of the tuner 302 into an analog signal; an analog-to-digital converter (ADC) 305 for converting the analog signal into a digital signal; a carrier recovery part 306 for converting the digital signal into a baseband signal; a DC limiter 307 for removing a pilot signal from an output signal of the carrier recovery part 306; a synchronizing part 308 for extracting a synchronizing signal from an output signal of the DC limiter 307 and recovering a symbol timing; a channel equalizer 309 for removing linear noise from the signal whose DC component is removed; a phase tracking part 310 for removing residual phase jitter from the signal whose linear noise is removed; and an FEC part 311 for decoding the signal, which is an operation opposite to the digital channel coding of the transmitter.
The tuner 302, the SAW filter 303 and the IF processing part 304 can be called an analog processing part, and the ADC 305 and the analog processing part can be called a digital processing part. Also, the DC limiter 307 and the synchronizing part 308 can be called a clock demodulation part 312, and the channel equalizer 309 and the phase tracking part 310 can be called a noise removing part 313.
That is, if a VBS-modulated RF signal is received through the antenna 301, the tuner 302 selects a desired channel frequency by using a heterodyne modulation system and then the VSB signal of the RF band carried on the channel frequency is lowered to a fixed IF band (generally, 44 MHz or 43.75 MHz) and signals of the other channels are properly filtered.
An output signal of the tuner 302 passes through the SAW filter 303, which removes signals of undesired bands and noise signals and serves as an analog matching filter.
For example, a digital broadcasting signal has all information within a band from the IF band of 44 MHz to a frequency band of 6 MHz, so that the SAW filter 303 removes all sections from the output signal of the tuner 302, except the band of 6 MHz in which information exists, and then outputs the remaining band signal to the IF processing part 304.
The IF processing part 304 converts the signal into an analog signal, and the ADC 305 converts the analog signal into a digital signal.
The passband signal converted into the digital signal is demodulated into a baseband signal by the carrier recovery part 306. In the baseband signal, a frequency of the pilot signal inserted for the carrier demodulation at the transmitter changes to 0 Hz, which is a DC component.
Since the DC component finished its role, the DC component is removed by the DC limiter 307.
Information of a synchronizing signal section is extracted by the synchronizing part 308. The information of the synchronizing signal section is used in the channel equalizer 309, the phase tracking part 310 and the FEC part 311.
The signal whose DC component is removed passes through the channel equalizer 309 to remove linear noises existing in the transport channel and the analog processing part of the receiver. Then, the signal passes through the phase tracking part 310 for removing the residual phase jitter and then it is decoded by the FEC part 311. If this process is finished, the digital TV receiver completes its function, and the transport stream equal to the signal inputted from the transmitter to the receiver is transmitted to a video/audio signal processing part (not shown).
FIG. 4 is a block diagram of the carrier recovery part of the digital TV receiver shown in FIG. 3.
In FIG. 4, the carrier recovery part 306 is implemented with a frequency phase locked loop (FPLL) proposed in the ATSC standard.
Referring to FIG. 4, if the passband analog signal is converted into the digital signal, a Hilbert transformer 402 shifts the signal by 90° such that the digital signal is transformed into a Q signal of an imaginary component. A delay unit 401 delays the digital signal by a predetermined time when the digital signal is transformed into the Q signal at the Hilbert transformer 402, and then outputs an I signal of a real component. A complex multiplier 403 multiplies the I and Q signals by an output signal of a voltage controlled oscillator (VCO) 410 to output a baseband I signal and a baseband Q signal. An FPLL includes a frequency locked loop (FLL) and a phase locked loop (PLL). The FLL includes an I signal low pass filter (LPF) 404, a delay unit 406, a code detector 407, a multiplier 408, a loop filter 409, and a VCO (or a numerically controlled oscillator (NCO)) 410. The FLL locks a frequency of the baseband I signal output from the complex multiplier 403. The PLL includes a Q signal LPF 405, the multiplexer 408, the loop filter 409 and the VCO 410. The PLL locks a frequency of the baseband Q signal output from the complex multiplier 403.
The I signal LPF 404, the delay unit 406 and the code detector 407 detect a frequency error, and the Q signal LPF 405 detects a phase error from the detected frequency error. Then, the multiplier 408 multiplies the frequency error and the phase error in order to obtain final frequency and phase error components (the controlled voltage).
The loop filter 409 removes RF components from the frequency and phase error components, and the VCO 410 converts an oscillation frequency according to the frequency and phase error components (the controlled voltage).
That is, the loop filter 409 filters only the baseband signal and the VCO 410 outputs the oscillation frequency varying according to the output signal of the loop filter 409. A beat frequency is removed by changing the frequency and phase of the carrier wave according to the varied oscillation frequency outputted from the VCO 410.
In the carrier recovery part 306, the I signal and the Q signal are demodulated and the frequency and phase are locked by separating the phase from the output signal of the SAW filter 303. Here, the center frequency of the VCO 410 is fixed to an intermediate frequency (for example, 46.690559 MHz) and the complex multiplier 403 multiplies the output of the VCO 410 and the output of the SAW filter 303 to thereby generate a baseband I channel signal i(t) and a baseband Q channel signal q(t).
At this point, the receiver can operate normally when the frequency of the pilot signal accurately is at the intermediate frequency (for example, 46.690559 MHz) at the output of the SAW filter 303. However, in many cases, the frequency of the pilot signal is not 46.690559 MHz.
Meanwhile, the output frequency of the VCO 410 is fixed to 46.690559 MHz. Thus, when the output frequency of the pilot signal is not 46.690559 MHz, there exists a beat frequency corresponding to a difference of two frequencies outputted from the complex multiplier 403. The FPLL is used to remove the beat frequency. That is, the frequency and phase of the carrier wave are changed due to the variation in the oscillation frequency of the VCO 410 and thus the beat frequency is removed. Accordingly, an object of the FPLL is to find a direction and magnitude of the movement of the oscillation frequency of the VCO 410.
The FPLL has a combination of a frequency locking look and a phase locking loop.
In FIG. 4, the frequency locking loop is configured with an auto frequency control filter (AFC), the code detector 407, the multiplier 408, the loop filter 409, the VCO 410 and the complex multiplier 403, and the phase locking loop is configured with the LPF 405, the multiplier 408, the loop filter 409, the VCO 410 and the complex multiplier 403.
FIGS. 5A to 5C are diagrams explaining a characteristic of the output signal of the complex multiplier and FIGS. 6A to 6C are diagrams explaining another characteristic of the output signal of the complex multiplier.
FIG. 5A shows a spectrum characteristic of the baseband I signal when the pilot signal is stably received, and FIGS. 5B and 5C show spectrum characteristics when the pilot signal component gets weaker while passing through the channel and therefore its position cannot be correctly found on the spectrum.
In the case of the FPLL that is dependent on the pilot, data component except the pilot component does not provide information necessary for the carrier recovery and also causes a jitter due to data after the carrier recovery. For these reasons, as shown in FIG. 4, the LPF is used to extract the pilot component from the received data.
When the pilot signal is weak due to the channel, it is preferable to use an LPF having a narrow bandwidth to extract the pilot signal from the data. However, if a carrier frequency offset exists due to the channel in such a state that the pilot signal is not weak, it is preferable to use an LPF having a wide bandwidth, as shown in FIGS. 5B and 5C. Meanwhile, if the component of the pilot signal is severely weak, even the LPF having the wide bandwidth cannot easily extract the pilot component.
FIG. 6A shows a spectrum characteristic of the baseband I signal when the pilot signal is stably received, and FIGS. 6B and 6C shows spectrum characteristics when the frequency offset exists in such a state that the power of the pilot signal is not weak and thus its position cannot be correctly found on the spectrum. As shown in FIGS. 6A to 6C, in case where the LPF having a narrow bandwidth is used, if the pilot signal is out of the narrow bandwidth, it is difficult to extract the pilot component.
According to the prior art, as described above, the LPF having the wide bandwidth is used to extract the pilot signal. Thus, when the frequency offset exists, no problem occurs. However, if the pilot signal is damaged due to the channel, power of the I signal from the complex multiplier becomes very weak near the DC. Therefore, in the system which performs the carrier recovery based on the pilot signal, its performance is degraded and thus the carrier cannot be recovered. If the LPF having the narrow bandwidth is simply used to extract the pilot signal, it is impossible to solve the problems occurring when the carrier frequency offset exists.